# Fixing broken references in schema

Sometimes a schema may refer to another schema document without indicating where the schema file can be found, like this:

## Schema reference without location

    <xs:import namespace="http://www.w3.org/1999/xlink" />

In other cases, a schema may refer to another schema on the network, which often slows down your compilation process and makes it unreliable. Yet in some other cases, a schema may reference another schema in relative path, and that may not match your directory structure.

XJC bundles a catalog resolver so that you can work around these situations without changing the schema documents. The main idea behind the catalog is "redirection" --- when XJC is about to fetch resources, it will consult the catalog resolver to see if it can find the resource elsewhere (which is usually your local resources.)

## Catalog format

The catalog resolver supports many different formats, but the easiest one is a line based `*.cat` format. Other than comments and empty lines, the file mainly consists of two kinds of declarations, `SYSTEM`, and `PUBLIC`.

### `sample-catalog.cat`

    --
      sample catalog file.

      double hyphens are used to begin and end a comment section.
    --

    SYSTEM "http://www.w3.org/2001/xml.xsd" "xml.xsd"

    PUBLIC "-//W3C//DTD XMLSCHEMA 200102//EN" "s4s/XMLSchema.dtd"

## Resolve by system ID

The SYSTEM entry has the format of "SYSTEM REFERENCE ACTUAL-LOCATION", which defines a simple redirection. Every time XJC loads any resource (be it schemas, DTDs, any entities referenced within), it will first resolve relative paths to absolute paths, then looks for a matching REFERENCE line. If it is found, the specified actual location is read instead. Otherwise XJC will attempt to resolve the absolutepath.

ACTUAL-LOCATION above accepts relative paths, and those are resolved against the catalog file itself (so in the above example, `xml.xsd` is assumed to be in the same directory with `sample-catalog.cat`.

What you need to be careful is the fact that the REFERENCE portion must be absolute, and when XJC finds a reference in schema, it will first convert that to the absolute path before checking the catalog. So what this means is that if your schema is written like this:

### Schema reference by relative path

    <xs:import namespace="http://www.w3.org/1999/xlink" schemaLocation="xlink.xsd" />

Then your catalog entry would have to look like this:

### xlink.cat

    -- this doesn't work because xlink.xsd will be turned into absolute path --
    SYSTEM "xlink.xsd" "http://www.w3.org/2001/xlink.xsd"

    -- this will work, assuming that the above schema is in /path/to/my/test.xsd --
    SYSTEM "/path/to/my/xlink.xsd" "http://www.w3.org/2001/xlink.xsd"

## Resolve by public ID / namespace URI

Another kind of entry has the format of "PUBLIC PUBLICID ACTUAL-LOCATION" or "PUBLIC NAMESPACEURI ACTUAL-LOCATION".

The "`PUBLICID`" version is used to resolve DTDs and entities in DTDs. But this type of entry is also used to resolve `<xs:import>` statements. XJC will match the value of the namespace attribute and see if there's any matching entry. So given a schema like this:

### Schema import

    <xs:import namespace="http://www.w3.org/1999/xlink" schemaLocation="xlink.xsd" />
    <xs:import namespace="http://www.w3.org/1998/Math/MathML" />

The following catalog entries will match them.

### by-publicid.cat

    PUBLIC "http://www.w3.org/1999/xlink" "http://www.w3.org/2001/xlink.xsd"
    PUBLIC "http://www.w3.org/1998/Math/MathML" "/path/to/my/mathml.xsd"

As you can see, XJC will check the PUBLIC entries regardless of whether `<xs:import>` has the schemaLocation attribute or not. As with the case with the SYSTEM entry, the ACTUAL-LOCATION part can be relative to the location of the catalog file.

## Specifying the catalog file

Once you write a catalog file, you'd need to specify that when you invoke XJC.

CLI

- To do this from the CLI, use the `-catalog` option. See `xjc -help` for more details.

Ant

- Use the catalog attribute on the `<xjc>` task. See XJC ant task documentation for more details.

Maven

- For the Maven plugin, use the `<catalog>` element in the configuration:

      <plugin>
          <groupId>org.jvnet.jaxb2.maven2</groupId>
          <artifactId>maven-jaxb2-plugin</artifactId>
          <configuration>
              <!-- relative to the POM file -->
              <catalog>mycatalog.cat</catalog>
          </copnfiguration>
      </plugin>

## Debugging catalog file

If you are trying to write a catalog file and banging your head against a wall because it's not working, you should enable the verbose option of the catalog resolver. How you do this depends on what interface you use:

CLI

- Specify `export XJC_OPTS="-Dxml.catalog.verbosity=999"` then run XJC.

Ant/Maven

- Add `-Dxml.catalog.verbosity=999` as a command line option to Ant/Maven.

If you are otherwise invoking XJC programmatically, you can set the above system property before invoking XJC.